Kohler Twin Starts easily but won't stay running

Not entirely a simple story.  My Kubota ZG123S had an SV820 engine on it.  Had about 275 hours when it suffered a lower seal failure and lost enough oil while cutting grass to basically kill the engine.  It went from good to bad rather quickly, and I shut it down promptly, but after that it was locked up.  So I picked up a CV730 block in usable condition and migrated most of the peripheral parts - flywheel, magneto, coils, carb, intake, fuel pump, etc so that parts I knew were working at the time of death for the 820 would be in play, as well as the engine wiring interface with the Kubota.  There were a few minor differences between the blocks, but all seemed to be insignificant...like the crankcase vent connection.  The carb has been cleaned, the fuel solenoid works, the float is fine.  The fuel pump appears to work fine....it filled a dry carb enough to start within 5 seconds of cranking.  And it runs like a champ....for about 4-5 seconds.  But, as eager as it is to start, it promptly shuts down.  It's not losing voltage at the fuel solenoid.  Perhaps I should have posted to the Kubota forum as well, but just wondering if anyone has been down a similar path.  As cheap as fuel pumps are, I may just buy one to see if it's a starvation issue.  Appreciate any thoughts from the group.

Yes, it will run on carb cleaner.  I've eliminated errant electrical issues, i.e. the coils don't get shorted as if turned off and the fuel solenoid remains on the whole time. Carb was very clean when I reassembled it.  Put new seals on the intake manifold.  As I said before, the fuel pump is moving gas, but perhaps not fast enough.  When it first starts, it runs too well to have a valve or head issue.  

A bit of commentary on the 'net indicates these vacuum fuel pumps are pretty weak on a good day, and it doesn't take much to disrupt adequate flow.  Putting a previously working fuel and electrical system on an older block seemed simple enough, but that might have been just enough to hit the tipping point of the pump's ability to pull through the filter.  Just picked up a new filter today.  Vacuum hose seems sound, no cracks, and the pump pulls gas when manipulated.  Unless it's somehow possible to have a weak vacuum on the valve cover (no oil leaks, gasket is good) I think the filter will be the fix.  If not, I will try gravity feeding fuel as a last test of the carb in its current state of clean.  

Problem has been solved.  The entire issue led me down several rabbit holes, all of which eliminated what it wasn't.  Put on a new fuel filter, and when that didn't solve it, I began looking at the differences between the engine blocks again.  Even though the manual for the CV730 showed the fuel pump being driven off valve cover vacuum, there was another illustration showing it being driven off of a crankcase vent on the block side opposite the starter.  I had routed that into the airbox, since the SV820 had routed a block vent into the airbox.  Put the crankcase vent into the fuel pump and the valve cover vent into the airbox, and it cranked right up and kept running.  Now if this FrankenKohler will just keep me going for the rest of the season and maybe a few more, I'll be a happy camper.